According to the current expansion of the personal computers used for multimedia information, liquid crystal displays (LADS) have become larger, and TUFT type liquid crystal displays providing high quality images have become the main current in the field of liquid crystal displays. For these displays, a major improvement in viewing angle characteristics is strongly demanded.
In order to meet this demand, In-Plane Switching (TIPS) mode and Vertically Aligned (VA) mode LADS have been developed in addition to conventional Twisted Nematic (TN) type LADS, and have already been put into practical use.
Furthermore, Optically Compensatory Bend (ORB) mode LADS which enable high speed switching of the image and exhibit superiority in showing motion pictures are also being commercialized. However, these types of displays, except for TIPS type, have problems in viewing angle characteristics and trials to improve the viewing angle characteristics by using an optical compensation sheet (hence, also referred to as an optical compensation film) are being carried out.
Descriptions regarding optical compensation sheets, to be used for the following modes of liquid crystal cells, are found in the following patent documents, for example: for TN (Twisted Nematic) mode LADS, Patent Documents 1-4; for TIPS (In-Plane Switching) mode and FCC (Ferroelectric Liquid crystal) mode LADS, Patent Document 5; for ORB (Optically Compensatory Bend) mode and THAN (Hybrid Aligned Nematic) mode LADS, Patent Documents 6 and 7; for STAN (Super Twisted Nematic) mode LADS, Patent Document 8; and for VA (Vertically Aligned) mode LADS, Patent Document 9.
The above mentioned optical compensation sheet (also referred to as an optical compensation film) is usually adhered to a polarizing plate and used as a monolithic elliptic polarizing plate.
Known as one of the examples of optical compensation sheets usable for this purpose is an optical compensation sheet for a TN mode liquid crystal display, which comprises stacked layers of a coated and immobilized disco tic liquid crystalline compound formed on a cellulose acetate support.
Another example is an optical compensation sheet for a TN mode liquid crystal display, which comprises stacked layers of an optically positive and uniaxial liquid crystalline compound formed on a cellulose acetate support.
Each of these optical compensation sheets has been prepared so as to exhibit desired optical compensation properties under a specified condition by controlling the in-plane retardation value (Ro) and the out-of-plane retardation value in the thickness direction (Rt) of the sheet by suitably selecting a cellulose ester support and liquid crystal layers.
Vertically Aligned mode liquid crystal displays (such as VA, MVA (multidomained VA) and PVA (patterned VA)) are of “normally black mode displays”, and enable higher contrast. These displays also enable higher switching speed of images compared to TN or TIPS mode displays. Accordingly, these displays are widely used for monitors and TVs. Since VA mode LADS by nature have narrow viewing angles in oblique directions (or being 45 degrees to the absorbing axis or a transmission axis of a polarizing plate), various types of optical compensation sheets or wide viewing angle films have been proposed to improve viewing angle characteristics, some of which have already been marketed.
For example, trials to prevent leakage of light in any direction using in combination of various λ/2 and λ/4 retardation plates have been proposed (for example, Patent Documents 10-14). However, the retardation plate to be used for this purpose requires having very high uniformity in retardation as is easily deduced from the very sensitive alignment angle to the polarizing plate, which results in a considerably low productivity. A very precise technique in adjusting the angle between the optical compensation sheet and the polarizing plate is also required, which also causes lowering of productivity. Accordingly, an easier method to effectively improve the viewing angle characteristics has been searched for.
In order to widen the viewing angle of a Vertically Aligned mode LCD, a method to use a retardation sheet, of which the in-plane delayed phase axis is placed orthogonal to the absorbing axis of a polarizing film, has been proposed.
This method was proposed in the early stages of the development of a Vertically Aligned ECB mode LCD (for example, in Patent Document 15), and a drastic improvement has been attained in viewing angle characteristics in oblique directions. The above improvement has been due to a design specifically focusing on broadening viewing angles.
A method to use a stacked layer type wide viewing angle film using a cholesterick liquid crystal compound has been proposed to widen the viewing angle while suppressing light leakage not only in the frontal direction but also in oblique directions (for example, in Patent Document 16).
As described above, a viewing angle of over 80 degrees at a wavelength of, for example, around 550 nm has been attained by use of various retardation films.
However, as the viewing angle in an oblique direction has exceeded 80 degrees, red hue in black areas in the display has become a pronounced problem, which had been tolerable at lesser viewing angles. The improvement in viewing angle characteristics has lead to the commercialization of large size liquid crystal displays, which resulted in a strong need for solving the red hue problem in the field of LADS. The red hue problem has been difficult to solve merely by optimizing the retardation value of optical film at a wavelength of 550 nm, or by adjusting the wavelength dispersion in the frontal viewing angle.                (Patent Document 1)                    Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 6-214,116.                        (Patent Document 2)                    U.S. Pat. No. 5,583,679                        (Patent Document 3)                    U.S. Pat. No. 5,646,703                        (Patent Document 4)                    DE No. 3,911,620(A1)                        (Patent Document 5)                    JP-A 10-54,982                        (Patent Document 6)                    U.S. Pat. No. 5,805,253                        (Patent Document 7)                    WO 96/37804                        (Patent Document 8)                    JP-A 9-26,572                        (Patent Document 9)                    Japanese Pat. No. 2,866,372                        (Patent Document 10)                    JP-A 5-11,356                        (Patent Document 11)                    JP-A 6-14,842                        (Patent Document 12)                    JP-A 2002-174,727                        (Patent Document 13)                    JP-A 2002-372,622                        (Patent Document 14)                    JP-A 2003-43,262                        (Patent Document 15)                    Examined Japanese Patent Publication No. 7-69536                        (Patent Document 16)                    JP-A 2002-182,212                        